Cell for the electrolytic production of aluminum



AugLZI, 1951 L. FERRAND} CELL FOR THE ELECTROLYTIC PRODUCTION OF ALUMINUM Original Filed July 16, 1946 Z,. 1 6,31 canal Patented Aug. 21 1951 i CELL'FOR THE ELECTROLYTIC PRODUC- TION OF ALUMINUM Louis Ferrand, Paris, France Original application July 16, 1946, Serial No. 684,066. Divided and this application January '13, 1948, Serial'No. 1,973. In France December now Patent No. 2,560,854.. That application is 4 concerned with various means for providing automatic operation of electrolytic cells, and more specifically for maintaining within narrow limits the temperature of the bath, as well as the percentage of material dissolved in said bath.

The attainment of continuous and automatic temperature regulation of cells for aluminum production is facilitated by providing for continuous addition of the alumina, and particularly in the case of large electrolytic cells in which the volume of the fused bath per unit of current is small. This is because the addition of large amounts of alumina periodically to the bath produces sudden reductions in the bath temperature, either on account of the sudden opening of the bath to the atmosphere or on account of the heat absorbed by the bulk of alumina which is introduced. The temperature drop from such causes in large electrolytic cells may be as high as 30 C.

Where provision is made for the automatic regulation of the bath temperature, as soon as a variation is detected in said temperature (and which is not the consequence of a variation in "the surrounding temperature or of some other accidental cause such as replacement of the anode or the tapping of the cell) it is possible to assume that the variation is due to a change in the efiiciency of the electrolytic process by which the thermo-chemical balance sheet is disturbed, which indicates that the rate of dissolution of the alumina should be accelerated or slowed down depending upon whether the bath temperature needs to be lowered or raised. Also, automatic temperature regulation enables the bath to be kept permanently in cool working condition and thus to secure the highest current efficiency, since it will be no longer necessary to provide for a margin of safety to prevent the difficulties attendant upon temperature changes incident to discontinuous admission of the alumina.

The present invention has for its object to provide a solution to the problem of continuous feeding of alumina, in the particular example chosen of aluminum production. The invention will best be understood from the following specification of a preferred embodiment of the invention, taken in connection with the appended drawing, in which:

Fig. 1 is an axial sectional view of a portion 4 Claims. (Cl. 204247) of an electrolytic cell showing the invention, and Fig. 2 is a perspective view of a portion :of the bottom of thealumina reservoir.

Referring now to Fig. lot the drawing each horizontal anode of the cell. is provided with its own alumina reservoir I which is closed bythe walls illustrated .by lines A, B, C, and ,D, and whose bottom indicated by letters E,.D, C,.H com.- prises a downwardly convex stainless steel sheet 2 which is polishedon its under surface and which has a parabolic profile so that heat :rays a, ,8 coming from the surface of the. anode will be reflected by the parabolic surface in the direction 5, 'y away from the focus F of the parabola, and will impinge on and bereflected once more bythe outwardly bulged walls 3 such asE-E, HI-I' of the gas bell. These 'wallsareconstituted by aluminum sheetspolished on-both surfaces and bolted in a gas-tight manner along :the

generatricies E and H of the parabolic steel sheet 2. Of course, these aluminum sheets will melt progressively as they sink into the liquid bath.

.As shown by the path of the reflected-rays 5 'Y in Fig. 1, it is possible with a suitable profile or shape of the parabolic surface of the bottom .2, to ensure that a substantial portion of the radiant heat emitted by the anode will be reflected into the space between the anode and be. made-of alloy steel which is resistant to the molten bath.

The gases that are evolved electrolytically all around the anode are collected by frusto-conical tubes 4 so dimensioned that at the beginning the rate of flow of the gases does not exceed 0.1 meter per second or thereabouts. Thegases are then cooled in a cylindrical receiver provided with fins 5 and in which they are stripped of any last trace of alumina (which may have been entrained by the gases) as by flowing through a comparatively fine-wire gauze sieve 6, for example a 250-mesh sieve for particles of grain size, prior to their being disposed of through the axial duct 1 by means of a compressor.

A branch pipe (not shown) may be provided, leading from the delivery side of the compressor to enable the alumina reservoir I to be kept under pressure for the purpose of facilitating the introduction of alumina into the bell through calibrated orifices provided in the bottom 2, so that if the reservoir unexpectedly becomes empty, the only result will be a back-flow into the bell of a part of the gas drawn therefrom.

Fig. 2 illustrates in detail the arrangement of the orifices In in a slide I9 arranged for longitudinal movement by appropriate means in a guide-way 20 provided in the strengthened twoply bottom 2 of the alumina reservoir. The bottom has an elongated aperture 1 I with which the small calibrated orifices It] will successively come into register as the slide I9 is moved. At constant pressure, the flow of alumina will thus be proportional to the number of orifices ID in register with aperture H. The dissolving of the alumina present in that portion of the free surface of the bath located between the anodes and the bell is also greatly facilitated by the bubbling of the gases from the electrolysis, by which said surface is kept in constant agitation.

A further way of controlling the feed of alumina in accordance with the efficiency of the electrolysis (and which is especially profitable if temperature regulation is provided for), consists in varying the gas pressure in reservoir I by proper manual or automatic actuation of the bypass valve of the exhaustor-compressor.

The alumina which is present on the ceiling 2 of the bell acts as a heat insulator and decreases the heat loss through said ceiling. The reservoir A, B, C, D, may be provided at one end thereof with a pressure-establishing pipe and at its other end with a further pipe through which the alumina is forced in at a pressure higher than that prevailing in the reservoir.

The above arrangements lend themselves very well to exhausting the gases from the electrolysis without any admixture of air. These gases, which are usually 50% CO and 50% CO2, may be subjected to a scouring step and then used in any synthetic process for which pure CO is required.

The anodes shown at the left and right extremities of Fig. 1 are respectively nearly all consumed and nearly new, to illustrate how, despite differences in anode elevation, it is possible to maintain a layer of alumina at least 15 cm. thick all around the gas bell, to act as a packing by means of which a pressure of at least 100 mm. of water (depending on the specific weight of the alumina used) can be maintained within the hell; that is, a vacuum more than sufficient for the collection of the gases without any admixture of air.

A further technical advantage of the arrangements described is that by reason of the considerably increased thickness of the layer of alumina covering" both the intervals between the anodes and the gas bell itself (40 cm. in the central portion thereof that serves as a reservoir), the bath is protected much better than in previously known free-anode cells. Thus, the dissolution of alumina is speeded up with a thinner solidified layer of the bath, including at the outside of the bell, and the heat losses are reduced by the decreased radiation, with resulting savings in power.

Having now particularly described and ascertained the nature of this invention and in what manner the same is to be performed, I declare that what I claim is:

1. In a cell for the electrolytic production of aluminum, at least one horizontal anode arranged to dip into a melt contained in said cell, a metallic gas bell surrounding said anode in spaced relation thereto and having its lateral walls bulged outwardly, said lateral walls being formed of polished aluminum and arranged to dip into the melt at their lower edges, and a downwardly-convex ceiling plate secured to said lateral walls above said anode and vertically spaced therefrom, and having its under surface polished to reflect toward the surface of the melt between said walls and said anode heat rays emanating upwardly therefrom.

2. A cell in accordance with claim 1, in which lateral walls extend upwardly above said ceiling plate to define a reservoir thereon for receiving a body of alumina, said ceiling plate being apertured to permit passage of alumina from said reservoir into said hell.

3. A cell in accordance with claim 1, including a central aperture in said ceiling plate, a tubular conduit extending upwardly from said aperture. and a gas-collecting housing in communication with said conduit.

4. A cell in accordance with claim 3, including means for admitting alumina into said bell, and in which said housing is provided with heatradiating fins and a filter for the collection of alumina dust entrained in gas entering said housing from said bell.

LOUIS FERRAND.

REFERENCES CITED UNITED STATES PATENTS Name Date Trematore Nov. 30, 1937 FOREIGN PATENTS Country Date France Jan. 16, 1883 Number Number 

1. IN A CELL FOR THE ELECTROLYTIC PRODUCTION OF ALUMINUM, AT LEAST ONE HORIZONTAL KANODE ARRANGED TO DIP INTO A MELT CONTAINED IN SAID CELL, A METALLIC GAS BELL SURROUNDING SAID ANODE IN SPACED RELATION THERETO AND HAVING ITS LATERAL WALLS BULGED OUTWARDLY, SAID LATERAL WALLS BEING FORMED OF POLISHED ALUMINUM AND ARRANGED TO DIP INTO THE MELT AT THEIR LOWER EDGES, AND A 